Artificial Ankle-Foot System with Spring, Variable-Damping, and Series-Elastic Actuator Components

ABSTRACT

An artificial foot and ankle joint consists of a curved leaf spring foot member having a heel extremity and a toe extremity, and a flexible elastic ankle member that connects the foot member for rotation at the ankle joint. An actuator motor applies torque to the ankle joint to orient the foot when it is not in contact with the support surface and to store energy in a catapult spring that is released along with the energy stored in the leaf spring to propel the wearer forward. A ribbon clutch prevents the foot member from rotating in one direction beyond a predetermined limit position. A controllable damper is employed to lock the ankle joint or to absorb mechanical energy as needed. The controller and sensing mechanisms control both the actuator motor and the controllable damper at different times during the walking cycle for level walking, stair ascent, and stair descent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/348,570, filed Jan. 11, 2012, which is a continuation of U.S. patentapplication Ser. No. 11/495,140, filed Jul. 29, 2006, which is anon-provisional of, and also claims the benefit of the filing date of,U.S. Provisional Patent Application Ser. No. 60/704,517 filed on Aug. 1,2005, each of which is herein incorporated by reference.

U.S. application Ser. No. 13/348,570 is a continuation in part of, andclaims the benefit of the filing date of, U.S. patent application Ser.No. 11/395,448 filed on Mar. 31, 2006. application Ser. No. 11/395,448was a non-provisional of, and claimed the benefit of the filing date of,U.S. Provisional Patent Application Ser. No. 60/666,876 filed on Mar.31, 2005 and U.S. Provisional Patent Application Ser. No. 60/704,517filed on Aug. 1, 2005, each of which is herein incorporated byreference.

This application incorporates the disclosures of each of the foregoingapplications herein by reference.

FIELD OF THE INVENTION

This invention relates generally to prosthetic devices and artificiallimb and joint systems, including robotic, orthotic, exoskeletal limbs,and more particularly, although in its broader aspects not exclusively,to artificial feet and ankle joints.

BACKGROUND OF THE INVENTION

In the course of the following description, reference will be made tothe papers, patents and publications presented in a list of referencesat the conclusion of this specification. When cited, each listedreference will be identified by a numeral within curly-braces indicatingits position within this list.

As noted in {1} {2} {3}, an artificial ankle-foot system ideally needsto fulfill a diverse set of requirements. The artificial system must bea reasonable weight and have a natural morphological shape, but stillhave an operational time between refueling or battery recharges of atleast one full day. The system must also be capable of varying itsposition, impedance, and motive power in a comparable manner to that ofa normal, healthy biological limb. Still further, the system must beadaptive, changing its characteristics given such environmentaldisturbances as walking speed and terrain variation. The embodiments ofthe invention that are described in this specification employ novelarchitectures capable of achieving these many requirements.

From recent biomechanical studies {1} {2} {3}, researchers havedetermined researchers have determined that early stance period anklestiffness varies from step-to-step in wag. Furthermore, researchers havediscovered that the human ankle performs more positive mechanical workthan negative work, especially at moderate to fast wag speeds {1} {2}{3}. The added ankle power is important for providing adequate forwardprogression of the body at the end of each stance period. Indistinction, for stair descent, the ankle behaves as a variable damperduring the first half of stance, absorbing impact energies {2}. Thesebiomechanical findings suggest that in order to mimic the actualbehavior of the human ankle, joint stiffness, motive power, and dampingmust be actively controlled in the context of an efficient, highcycle-life, quiet and cosmetic ankle-foot artificial joint.

For level ground ambulation, the ankle behaves as a variable stiffnessdevice during the early to midstance period, storing and releasingimpact energies. Throughout terminal stance, the ankle acts as a torquesource to power the body forward. In distinction, the ankle variesdamping rather than stiffness during the early stance period of stairdescent. These biomechanical findings suggest that in order to mimic theactual behavior of a human joint or joints, stiffness, damping, andnonconservative, motive power must be actively controlled in the contextof an efficient, high cycle-life, quiet and cosmetic biomimetic limbsystem, be it for a prosthetic or orthotic device. This is also the casefor a biomimetic robotic limb since it will need to satisfy the samemechanical and physical laws as its biological counterpart, and willbenefit from the same techniques for power and weight savings.

In the discussion immediately below, the biomechanical properties of theankle will be described in some detail to explain the insights that haveguided the design and development of the specific embodiments of theinvention and to define selected terms that will be used in thisspecification.

Joint Biomechanics: The Human Ankle

Understanding normal walking biomechanics provides the basis for thedesign and development of the artificial ankle joint and ankle-footstructures that embody the invention. Specifically, the function ofhuman ankle under sagittal plane rotation is described below fordifferent locomotor conditions including level-ground walking andstair/slope ascent and descent. From these biomechanical descriptions,the justifications for key mechanical components and configurations ofthe artificial ankle structures and functions embodying the inventionmay be better understood.

Level-Ground Walking

A level-ground walking gait cycle is typically defined as beginning withthe heel strike of one foot and ending at the next heel strike of thesame foot {8}. The main subdivisions of the gait cycle are the stancephase (about 60% of the cycle) and the subsequent swing phase (about 40%of the cycle) as shown in FIG. 1. The swing phase represents the portionof the gait cycle when the foot is off the ground. The stance phasebegins at heel-strike when the heel touches the floor and ends attoe-off when the same foot rises from the ground surface. Additionally,we can further divide the stance phase into three sub-phases: ControlledPlantar flexion (CP), Controlled Dorsiflexion (CD), and Powered Plantarflexion (PP).

Each phase and the corresponding ankle functions which occur whenwalking on level ground are illustrated in FIG. 1. The subdivisions ofthe stance phase of walking, in order from first to last, are: theControlled Plantar flexion (CP) phase, the Controlled Dorsiflexion (CD)phase, and the Powered Plantar flexion (PP) phase.

CP begins at heel-strike illustrated at 103 and ends at foot-flat at105. Simply speaking, CP describes the process by which the heel andforefoot initially make contact with the ground. In {1, 12}, researchersshowed that CP ankle joint behavior was consistent with a linear springresponse where joint torque is proportional to joint position. Thespring behavior is, however, variable; joint stiffness is continuouslymodulated by the body from step to step.

After the CP period, the CD phase continues until the ankle reaches astate of maximum dorsiflexion and begins powered plantarflexion PP asillustrated at 107 Ankle torque versus position during the CD period canoften be described as a nonlinear spring where stiffness increases withincreasing ankle position. The main function of the ankle during CD isto store the elastic energy necessary to propel the body upwards andforwards during the PP phase {9} {3}.

The PP phase begins after CD and ends at the instant of toe-offillustrated at 109. During PP, the ankle can be modeled as a catapult inseries or in parallel with the CD spring or springs. Here the catapultcomponent includes a motor that does work on a series spring during thelatter half of the CD phase and/or during the first half of the PPphase. The catapult energy is then released along with the spring energystored during the CD phase to achieve the high plantar flexion powerduring late stance. This catapult behavior is necessary because the workgenerated during PP is more than the negative work absorbed during theCP and CD phases for moderate to fast walking speeds {1} {2} {3} {9}.

During he swing phase, the final 40% of the gait cycle, which extendsfrom toe-off at 109 until the next heel strike at 113, the foot islifted off the ground.

Stair Ascent and Descent

Because the kinematic and kinetic patterns at the ankle during stairascent/descent are significantly different from that of level-groundwalking {2}, a separate description of the ankle-foot biomechanics ispresented in FIGS. 2 and 3.

FIG. 2 shows the human ankle biomechanics during stair ascent. The firstphase of stair ascent is called Controlled Dorsiflexion 1 (CD 1), whichbegins with foot strike in a dorsiflexed position seen at 201 andcontinues to dorsiflex until the heel contacts the step surface at 203.In this phase, the ankle can be modeled as a linear spring.

The second phase is Powered Plantar flexion 1 (PP 1), which begins atthe instant of foot flat (when the ankle reaches its maximumdorsiflexion at 203) and ends when dorsiflexion begins once again at205. The human ankle behaves as a torque actuator to provide extraenergy to support the body weight.

The third phase is Controlled Dorsiflexion 2 (CD 2), in which the ankledorsiflexes until heel-off at 207. For the CD 2 phase, the ankle can bemodeled as a linear spring.

The fourth and final phase is Powered Plantar flexion 2 (PP 2) whichbegins at heel-off 207 and continues as the foot pushes off the step,acting as a torque actuator in parallel with the CD 2 spring to propelthe body upwards and forwards, and ends when the toe leaves the surfaceat 209 to being the swing phase that ends at 213.

FIG. 3 shows the human ankle-foot biomechanics for stair descent. Thestance phase of stair descent is divided into three sub-phases:Controlled Dorsiflexion 1 (CD1), Controlled Dorsiflexion 2 (CD2), andPowered Plantar flexion (PP).

CD1 begins at foot strike illustrated at 303 and ends at foot-flat 305.In this phase, the human ankle can be modeled as a variable damper. InCD2, the ankle continues to dorsiflex forward until it reaches a maximumdorsiflexion posture seen at 307. Here the ankle acts as a linearspring, storing energy throughout CD2. During PP, which begins at 307,the ankle plantar flexes until the foot lifts from the step at 309. Inthis final PP phase, the ankle releases stored CD2 energy, propellingthe body upwards and forwards. After toe-off at 309, the foot ispositioned controlled through the swing phase until the next foot strikeat 313.

For stair ascent depicted in FIG. 2, the human ankle-foot can beeffectively modeled using a combination of an actuator and a variablestiffness mechanism. However, for stair descent, depicted in FIG. 3, avariable damper needs also to be included for modeling the ankle-footcomplex; the power absorbed by the human ankle is much greater duringstair descent than the power released by 2.3 to 11.2 J/kg {2}. Hence, itis reasonable to model the ankle as a combination of a variable-damperand spring for stair descent {2}.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention take the form of anartificial ankle system capable of providing biologically-realisticdynamic behaviors. The key mechanical components of these embodiments,and their general functions, may be summarized as follows:

1. One or more passive springs—to store and release elastic energy forpropulsion;

2. One or more series-elastic actuators (muscle-tendon)—to control theposition of the ankle, provide additional elastic energy storage forpropulsion, and to control joint stiffness; and

3. One or more variable dampers—to absorb mechanical energy during stairand slope descent.

The above-identified U.S. patent application Ser. No. 11/395,448 filedon Mar. 31, 2006 describes related artificial limbs and joints thatemploy passive and series-elastic elements and variable-dampingelements, and in addition employ active motor elements in arrangementscalled “Biomimetic Hybrid Actuators” forming biologically-inspiredmusculoskeletal architectures. The electric motor used in the hybridactuators supply positive energy to and store negative energy from oneor more joints which connect skeletal members, as well as elasticelements such as springs, and controllable variable damper components,for passively storing and releasing energy and providing adaptiveimpedance to accommodate level ground walking as well as movement onstairs and surfaces having different slopes.

As described in application Ser. No. 11/395,448, an artificial ankle mayemploy an elastic member operatively connected in series with the motorbetween the shin member and the foot member to store energy when therelative motion of the foot and shin members is being arrested by acontrollable variable damping element and to thereafter apply anadditional torque to the ankle joint when the variable damping elementno longer arrests the relative motion of the two members.

As further described in application Ser. No. 11/395,448, An artificialankle may include an elastic member operatively connected in series withthe motor between the shin and foot members to store energy when thefoot member is moved toward the shin member and to release energy andapply an additional torque to the ankle joint that assists the motor tomove the foot member away from the shin member. A controllable dampingmember may be employed to arrest the motion of the motor to control theamount of energy absorbed by the motor when the foot member is movedtoward the shin member.

The Flex-Foot, made by Ossur of Reykjavik, Iceland, is a passivecarbon-fiber energy storage device that replicates the ankle joint foramputees. The Flex-Foot is described in U.S. Pat. No. 6,071,313 issuedto Van L. Phillips entitled “Split foot prosthesis” and in Phillips'earlier U.S. Pat. Nos. 5,776,205, 5,514,185 and U.S. Pat. No. 5,181,933,the disclosures of which are incorporated herein by reference. TheFlex-foot is a foot prosthesis for supporting an amputee relative to asupport surface and consists of a leaf spring having multiple flexingportions configured to flex substantially independently of one anothersubstantially completely along their length. The Flex-Foot has anequilibrium position of 90 degrees and a single nominal stiffness value.In the embodiments described below, a hybrid actuator mechanism of thekind described in the above-noted application Ser. No. 11/395,448 isused to augment a flexing foot member such as the Flex-Foot by allowingthe equilibrium position to be set to an arbitrary angle by a motor andlocking, or arresting the relative movement of, the foot member withrespect to the shin member using a clutch or variable damper.Furthermore, the embodiment of the invention to be described can alsochange the stiffness and damping of the prosthesis dynamically.

Preferred embodiments of the present invention take the form of anartificial ankle and foot system in which a foot and ankle structure ismounted for rotation with respect to a shin member at an ankle joint.The foot and ankle structure preferably comprises a curved flexibleelastic foot member that defines an arch between a heel extremity and atoe extremity, and a flexible elastic ankle member that connects saidfoot member for rotation at the ankle joint. A variable damper isemployed to arresting the motion of said foot and ankle structure withrespect to said shin member under predetermined conditions, andpreferably includes a stop mechanism that prevents the foot and anklestructure from rotating with respect to the shin member beyond apredetermined limit position. The variable damper may further include acontrollable damper, such as a magnetorheological (MR) brake, whicharrests the rotation of the ankle joint by controllable amount atcontrolled times during the walking cycle. Preferred embodiments of theankle and foot system further include an actuator motor for applyingtorque to the ankle joint to rotate said foot and ankle structure withrespect to said shin member.

In addition, embodiments of the invention may include a catapultmechanism comprising a series elastic member operatively connected inseries with the motor between the shin member and the foot and anklestructure. The series elastic member stores energy from the motor duringa first portion of each walking cycle and then releases the storedenergy to help propel the user forward over the walking surface at alater time in each walking cycle. The preferred embodiments of theinvention may employ a controller for operating both the motor and thecontrollable damper such that the motor stores energy in the serieselastic member as the shin member is being arrested by the controllabledamper.

The actuator motor which applies torque to the ankle joint may beemployed to adjust the position of the foot and ankle structure relativeto the shin member when the foot and ankle member is not in contact witha support surface. Inertial sensing means are preferably employed todetermine the relative elevation of the foot and angle structure and toactuate the motor in response to changes in the relative elevation,thereby automatically positioning the foot member for toe firstengagement if the wearer is descending stairs.

These and other features and advantages of the present invention will bebetter understood by considering the following detailed description oftwo illustrative embodiments of the invention. In course of thisdescription, frequent reference will be made to the attached drawings,which are briefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the different phases of a walking cycle experiencedby a human ankle and foot during level ground walking;

FIG. 2 depicts the phases of a walking cycle experienced by a humanankle and foot when ascending stairs;

FIG. 3 depicts the phases of a walking cycle experienced by a humanankle and foot during stair descent;

FIG. 4 shows the mechanical design of an anterior view of embodiment 1;

FIG. 5 shows a posterior view of embodiment 1;

FIG. 6 shows a side elevational view of embodiment 1;

FIG. 7 is a schematic depiction of embodiment 1;

FIG. 8 depicts a lumped parameter model of embodiment 1;

FIGS. 9-12 show the control sequence for embodiment 1 during groundlevel walking;

FIGS. 13-15 show the control sequence for embodiment 1 during stairascent;

FIGS. 16-19 show the control sequence for embodiment 1 during stairdescent;

FIG. 20 shows the mechanical design of an anterior view of embodiment 2;

FIG. 21 shows a posterior view of embodiment 2;

FIG. 22 shows a side elevational view of embodiment 2;

FIG. 23 is a schematic depiction of embodiment 2;

FIG. 24 depicts a lumped parameter model of embodiment 2;

FIGS. 25-28 show the control sequence for embodiment 2 during groundlevel walking;

FIG. 29 is a schematic block diagram of a sensing and control mechanismused to control the operation of the motors and dampers in ankle footsystems embodying the invention.

DETAILED DESCRIPTION

Two embodiments of an ankle-foot system contemplated by the presentinvention are described in detail below. The first embodiment(Embodiment 1) provides for elastic energy storage, variable-damping anda variable-orientation foot control. In addition to these capabilities,the second embodiment to be described includes a motor in series with aspring for providing joint spring stiffness control during the CP and CDphases, and a motive torque control during the PP phase of the walkingcycle as described above.

Embodiment 1 Mechanical Components

The mechanical design of embodiment 1 is seen in FIGS. 4-6 and thecorresponding schematic and lumped parameter model of embodiment 1 areshown in FIGS. 7 and 8, respectively. As seen in the side elevation viewof FIG. 6, there are four main mechanical elements in this embodiment:an elastic leaf spring structure 601, a dorsiflexion clutch (RibbonStop) seen at 603, a variable damper (MR brake) seen at 605, and anactuator system comprising a small motor seen at 607. As seen in theschematic of FIG. 7, these four main mechanical elements are shown as anelastic leaf spring structure 701, a dorsiflexion clutch (Ribbon Stop)703, a variable damper 705, and a motor actuator system 707.

The elastic leaf spring seen at 601 and 701 can be made from alightweight, efficient spring material such as carbon composite,fiberglass or a material of similar properties. As seen in FIG. 6, andas described in Phillips' U.S. Pat. No. 6,071,313 issued on Jun. 6,2000, the elastic leaf spring structure includes a heel portion seen at609 and a toe portion seen at 660. A curved, flexible ankle section 680is attached at its upper end to a brake mount member 690 which is mountsthe flexible foot for rotation about the axis of the ankle joint which,in FIG. 6, is located at the center of the MR brake 605.

The variable-damper mechanism seen at 605 and 705 can be implementedusing magnetorheological (MR), electrorheological (ER), dry magneticparticles, hydraulic, pneumatic, friction, or any similar strategy tocontrol joint damping. For embodiment 1, a MR system is employed. HereMR fluid is used in the shear mode where a set of rotary plates shearthin layers of MR fluid. When a magnetic field is induced across the MRlayers, iron particles suspended in carrier fluid form chains,increasing the shear viscosity and joint damping.

The ribbon stop seen at 603 and 703 prevents the ankle joint fromdorsiflexing beyond a certain maximum dorsiflexion limit, ranging from 0to 30 degrees depending on ankle performance requirements. The ribbonstop is uni-directional, preventing dorsiflexion but not impedingplantarflexion movements.

The actuator motor seen at 607 and 707 is a small, low-powerelectromagnetic motor that provides foot orientation control. The motorcan exert a torque about the ankle joint (indicated at 711) tore-position the foot (the elastic leaf spring 601, 701) relative to theshank depicted at 713 when the foot is not in contact with the ground.As seen in FIGS. 4-6, the shank frame for the ankle-foot assemblyattaches to a shin member (not shown) using a standard pyramid mountseen at 613 which may be used to attach the shank frame to the shinportion of an artificial limb or the wearer's stump. As will beunderstood, both of the artificial foot and ankle joint embodimentsdescribed in this specification may be used in combination withartificial limb structures such as the artificial knees and hipsdescribed in the above-noted U.S. patent application Ser. No.11/395,448.

Control System

For a better understanding of the control sequence of the artificialankle, a simplified 1D lumped parameter model of embodiment 1 seen inFIG. 8 is used to explain the behavior of the ankle-foot system underdifferent walking conditions.

From FIG. 7, it may be noted that the bending angle of the elastic leafspring 701 is independent of the ankle angle of the pin joint, thereforethe lumped parameter model includes two degrees of freedom: one for thedisplacement of the foot, X₁, and the other for the displacement of theshank X₂ as shown in FIG. 8. The leaf spring structure, seen at 601 inFIG. 6 and at 701 in FIG. 7, is modeled as a nonlinear spring shown at801 in FIG. 8 with a stiffness that varies with X₁, the foot bendingangle (displacement of the foot). The actuator motor seen at 807, thevariable-damper 805, and the ribbon stop seen at 803 act between themass of the shank at 820 and the mass of the foot at 830. The loadingforce F_(load)(t) due to body weight varies dynamically during thestance phase of each gait cycle.

Level-Ground Walking

The control sequence of Embodiment 1 for level-ground walking isdepicted in FIGS. 9-12. During level-ground walking, the variable-damperis set at a high damping level to essentially lock the ankle jointduring early to midstance, allowing the leaf spring structure to storeand release elastic energy. Once a critical dorsiflexion angle isachieved (between 0 to 30 degrees), the ribbon stop becomes taughtduring the remainder of the CD phase. When the ribbon is engaged, theleaf spring and shank can be treated as one single component because theribbon behaves as a clutch (FIG. 10). From heel strike to maximumdorisflexion, the leaf spring structure stores elastic energy (Ax, 0,&₂=0). In PP, as the loading from the body weight decreases, the springstructure releases its stored elastic energy, rotating in a plantarflexion direction and propelling the body upwards and forwards (FIG.11). After toe-off, the actuator controls the equilibrium position ofthe foot to achieve foot clearance during the swing phase and tomaintain a proper landing of the foot for the next gait cycle (FIG. 12).

The state of each element of the ankle-foot system during the fourphases of a level ground walking cycle are listed below:

-   -   Controlled Plantar Flexion (FIG. 9)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is OFF        -   3. Damper is ON        -   4. Leaf spring heel portion at 609 is being compressed    -   Controlled Dorsiflexion (FIG. 10)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 660 is being compressed    -   Powered Plantar Flexion (FIG. 11)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring ankle section 660 is releasing energy    -   Swing Phase (FIG. 12)        -   1. Actuator motor is ON (changing foot orientation)        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Foot leaf spring is slack

The maximum dorsiflexion ankle torque during level-ground walking is inthe range from 1.5 Ng to 2 Nm/kg, i.e. around 150 Nm for a 100 kg person{2}. With current technology, a variable-damper that can provide suchhigh damping torque and additionally very low damping levels isdifficult to build at a reasonable weight and size. Fortunately, themaximum controlled plantar flexion torque is small, typically in therange of 0.3 Nm/kg to 0.4 Ng. Because of these factors, a ribbon stopthat engages at a small dorsiflexion angle such as 5 degrees would lowerthe peak torque requirements of the variable-damper since the peakcontrolled plantar flexion torque is considerably smaller than the peakdorsiflexion torque.

During stair descent/downhill walking, the human ankle behaves like adamper from foot strike to 90° of dorsiflexion {11}. Beyond that, theankle behaves like a non-linear spring, storing elastic energy duringcontrolled dorsiflexion. Taking advantage of the biomechanics of thehuman ankle, it is reasonable to add a passive clutch for resistingdorsiflexion movements beyond 90°, thus allowing for a smaller sizedvariable damper. A ribbon stop is preferred as a unidirectional clutchbecause it is lightweight with considerable strength in tension.

Stair Ascent

FIGS. 13-15 depict the control sequence of embodiment 1 for stairascent. It is noted here that there are only three control phases/modesfor stair ascent, although the gait cycle for stair ascent can bedivided into 5 sub-phases, including Controlled Dorsiflexion 1 (CD1),Powered Plantarflexion 1 (PP1), Controlled Dorsiflexion 2 (CD2), PoweredPlantarflexion 1 (PP1), and Swing Phase. The main reason is that interms of control, we can combine phases PP1, CD2, and PP2 into onesingle phase since all three phases may be described using the samecontrol law. For ascending a stair, the clutch is engaged and the leafspring is compressed throughout ground contact (FIG. 13) because the toestrikes the ground first, engaging the ribbon stop during CD (Ax, 0,&₂=0). After the heel strikes the ground and then lifts off the ground,the toe leaf spring begins releasing its energy, supplying forwardpropulsion to the body (FIG. 14). The variable damper may be activatedto control the process of energy release from the leaf spring, but ingeneral, the damper is turned off so that all the stored elastic energyis used to propel the body upwards and forwards (Ax, 0, &₂ 0). Aftertoe-off, the actuator controls the equilibrium position of the ankle inpreparation for the next step (FIG. 15).

The state of each element of the ankle-foot system during these threephases of a stair ascent are listed below:

-   -   Controlled Dorsiflexion (FIG. 13)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 660 is being compressed    -   Powered Plantar Flexion (FIG. 14)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 660 is releasing energy    -   Swing Phase (FIG. 15)        -   1. Actuator motor is ON (changing foot orientation)        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Foot leaf spring is slack

Stair Descent

The control sequence for embodiment 1 for stair descent is depicted inFIGS. 16-19. After forefoot contact, the body has to be lowered untilthe heel makes contact with the stair tread {11} (FIG. 16). Therefore,the variable damper is activated as energy is dissipated duringcontrolled dorsiflexion (ΔX₁<=0, ΔX₂<=0). As is shown in FIG. 17, whenthe foot becomes flat on the ground, the ribbon stop becomes taut,compressing the toe leaf spring (ΔX₁<=0, ΔX₂=0). During PP, the toe leafspring releases its energy, propelling the body upwards and forwards(FIG. 18).

The state of each element of the ankle-foot system during the fourphases of stair descent are listed below:

-   -   Controlled Dorsiflexion 1 (FIG. 16)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is OFF        -   3. Damper is ON        -   4. Leaf spring toe section 660 is being compressed    -   Controlled Dorsiflexion 2 (FIG. 17)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 660 is being compressed    -   Powered Plantar Flexion (FIG. 18)        -   1. Actuator motor is OFF        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 660 is releasing energy    -   Swing Phase (FIG. 19)        -   1. Actuator motor is ON (changing foot orientation)        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Foot leaf spring is slack

Sensing for Embodiment 1

The ankle foot system preferably employs an inertial navigation system(INS) for the control of an active artificial ankle joint to achieve amore natural gait and improved comfort over the range of human walkingand climbing activities.

To achieve these advantages, an artificial ankle joint must becontrolled to behave like a normal human ankle For instance, duringnormal level ground walking, the heel strikes the ground first; but whendescending stairs, it is the toe which first touches the ground. Walkingup or down an incline, either the toe or the heel may strike the groundfirst, depending upon the steepness of the incline.

A difficult aspect of the artificial ankle control problem is that theankle joint angle must be established before the foot reaches theground, so that the heel or toe will strike first, as appropriate to theactivity. Reliable determination of which activity is underway while thefoot is still in the air presents implacable difficulties for sensorsystems presently employed on lower leg artificial devices.

The present invention addresses this difficulty by attaching an inertialnavigation system below the knee joint, either on the lower leg segmentor on the artificial foot. This system is then used to determine thefoot's change in elevation since it last left the ground. This change inelevation may be used to discriminate between level ground walking anddescending stairs or steep inclines. The ankle joint angle may then becontrolled during the foot's aerial phase to provide heel strike forlevel ground walking or toe strike upon detection of negative elevation,as would be encountered descending stairs or walking down a steepincline.

Inertial navigation systems rely upon accelerometers and gyroscopesjointly attached to a rigid assembly to detect the assembly's motion andchange of orientation. In accordance with the laws of mechanics, thesechanges may be integrated to measure changes of the system's positionand orientation, relative to its initial position and orientation. Inpractice, however, it is found that errors of the accelerometers andgyros produce ever-increasing errors in the system's estimated position.Inertial navigation systems can address this problem in one of two ways:by the use of expensive, high precision accelerometers and gyroscopes,and by incorporating other, external sources of information aboutposition and orientation, for instance GPS, to augment the purelyinertial information. But using either of these alternatives would makethe resulting system unattractive for an artificial ankle device.

However, we have found that an unaugmented, purely inertial system basedon available low cost accelerometers and rate gyros can providesufficiently accurate trajectory information to support proper controlof the angle of an actuated artificial ankle system.

An Illustrative Control Algorithm

Control of an actuated artificial ankle joint may be implemented asfollows:

-   -   A. During the foot flat (controlled dorsiflexion) phase of the        walking cycle, reset and maintain the measured elevation to        zero. When the foot is flat on the ground, its velocity and        acceleration are zero. Thus, this particular foot posture serves        as a reset point for the integration of angular and linear        velocities in the estimation of absolute positions.    -   B. During the push off phase, when powered plantarflexion        begins, measure the upward and downward movements to determine        the current elevation relative to the initial zero elevation        during the flat foot phase;    -   C. As long as the elevation remains above zero, maintain the        foot orientation that will provide heelstrike; and    -   D. If the elevation decreases below zero, reorient the angle        ankle to provide toe-first contact.

The foot flat phase may be detected by the absence of non-centrifugal,non-gravitational, linear acceleration along the length axis of thelower leg. Push off phase may be detected by the upward accelerationalong the axis of the lower leg. Elevation>0 and elevation <0 phases arerecognized from the change in relative elevation computed by the INSsince the end of foot flat phase.

Embodiment 2 Mechanical Design

The mechanical design of Embodiment 2 is shown in FIGS. 20-23. As seenin FIG. 22, the foot and ankle system includes an elastic leaf springstructure that provides a heel spring as seen at 2201 and a toe springas seen at 2206, the elastic leaf spring structure attaches to a brakemount member 2202 that rotates with respect to an ankle joint shankframe 2203 and a tibial side bracket 2204 about a pivot axis at thecenter of the MR brake seen at 2205. The actuator motor 2207 is mountedwithin the tibial side bracket 2204 and its drive shaft is coupledthrough a drive gear (not shown) to rotate the elastic leaf springstructure 2201 and 2206 with respect to the shank frame 2203 and sidebracket 2204 about the ankle joint. A catapult mechanism to providepowered plantar flexion during late stance is employed that consists ofa series elastic spring element seen at 2210 having an internal slider2212 that attaches to the brake mount 2202 at the lower actuator mount2213, and the spring element 2210 attaches to the upper actuator mount2216 at the top of the tibial side bracket 2204. A standard pyramidmount 2230 at the top of the tibial side bracket 2294 provides aconnection to the shin member (not shown).

The corresponding schematic of Embodiment 2 is seen in FIG. 23 and issimilar to that of Embodiment 1, including the heel and toe leaf spring2301, variable damper 2305, and ribbon stop 2303. The series elasticspring element is seen at 2310 connected in series with the actuatormotor 2307 to form the catapult.

One of the main challenges in the design of an artificial ankle is tohave a relatively low-mass actuation system that can provide a largeinstantaneous output power upwards of 200 Watts during Powered PlantarFlexion (PP) {2,11} Fortunately, the duration of PP is only 15% of theentire gait cycle, and the average power output of the human ankleduring the stance phase is much lower than the instantaneous outputpower during PP. Hence, a catapult mechanism is a compelling solution tothis problem.

The catapult mechanism is mainly composed of three components: anactuator motor, a variable damper and/or clutch and an energy storageelement. The actuator can be any type of motor system, includingelectric, shape memory alloy, hydraulic or pneumatic devices, and theseries energy storage element can be any elastic element capable ofstoring elastic energy when compressed or stretched. The damper can beany type of device including hydraulic, magnetorheological, pneumatic,or electrorheological.

With the parallel damper seen at 2305 in FIG. 23 activated to a highdamping level or with the parallel clutch 2303 activated, the serieselastic spring element 2310 can be compressed or stretched by theactuator 2307 in series to the spring 2310 without the joint rotating.The spring 2310 will provide a large amount of instantaneous outputpower once the parallel damping device 2305 or clutch 2303 isdeactivated, allowing the elastic element 2310 to release its energy. Ifthe actuator 2307 has a relatively long period of time to compress orstretch the elastic element 2310, its mass can be kept relatively low,decreasing the overall weight of the artificial ankle device. InEmbodiment 2, the catapult system comprises a magnetorheologicalvariable damper 2305 placed in parallel to the series elastic electricmotor system.

Control System

The lumped parameter model of Embodiment 2 is shown in FIG. 24. It isbasically the same as the model of Embodiment 1 as depicted in FIG. 8,except that we now place a spring element 2410 in series with theactuator 2407 and the foot mass structure 2430. The main idea here isthat if the variable MR damper seen at 2405 outputs high damping,locking the ankle joint, the foot and the shank become one singlecomponent. Once the joint is locked, the actuator 2407 compresses orstretches the spring element 2310. Once joint damping is minimized, thespring element 2410 will then push against the shank 2420 to provideforward propulsion during powered plantar flexion.

The control sequence of Embodiment 2 for level-ground walking will bediscussed in the next section. Stair ascent/descent can be deduced fromthe earlier descriptions for embodiment 1, and thus, will not bedescribed herein.

Level-Ground Walking

The control sequence of Embodiment 2 for level-ground walking isdepicted in FIGS. 25-28. During CP, the actuator controls the stiffnessof the ankle by controlling the displacement of the series spring (FIG.25). During CD, the toe carbon fiber leaf spring 2206 is compressed dueto the loading of body weight, while the actuator compresses the seriesspring to store additional elastic energy in the system (FIG. 26). Inthis control scheme, inertia and body weight hold the joint in adorsiflexed posture, enabling the motor to elongate the series spring.In a second control approach, where body weight and inertia areinsufficient to lock the joint, the MR variable damper would output ahigh damping value to essentially lock the ankle joint while the motorstores elastic energy in the series spring. Independent of the catapultcontrol approach, during PP as seen in FIG. 27, as the load from bodyweight decreases, both the leaf spring and the series catapult springbegin releasing stored elastic energy, supplying high ankle outputpowers. After toe-off, the actuator controls the position of the footwhile both the series spring and the leaf springs are slack as depictedin FIG. 28.

The state of each element of Embodiment 2 of the ankle foot systemduring the four phases of a level ground walking cycle are listed below:

-   -   Controlled Plantar Flexion (FIG. 25)        -   1. Actuator motor is ON        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Leaf spring heel portion at 2201 is being compressed    -   Controlled Dorsiflexion (FIG. 26)        -   1. Actuator motor is ON        -   2. Ribbon clutch is ON        -   3. Damper is OFF        -   4. Leaf spring toe section 2206 is being compressed    -   Powered Plantar Flexion (FIG. 27)        -   1. Actuator motor is ON        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Leaf spring toe section 2206 is releasing energy    -   Swing Phase (FIG. 28)        -   1. Actuator motor is ON (changing foot orientation)        -   2. Ribbon clutch is OFF        -   3. Damper is OFF        -   4. Foot leaf spring structure is slack

Sensing for Embodiment 2

As with Embodiment 1, an inertial navigation system for the control ofthe active artificial ankle joint will be employed to achieve a morenatural gait and improved comfort over the range of human walking andclimbing activities. The manner in which these navigation sensors willbe used is similar to that described for Embodiment 1.

Sensing and Control

As described above, investigations of the biomechanics of human limbshave revealed the functions performed by the ankle during normal walkingover level ground, and when ascending or descending a slope or stairs.As discussed above, these functions may be performed in an artificialankle joint using motors to act as torque actuators and to position thefoot relative to the shin member during a specific times of walkingcycle, using springs in combination with controllable dampers to act aslinear springs and provide controllable damping at other times in thewalking cycle. The timing of these different functions occurs during thewalking cycle at times described in detail above. The specificmechanical structures, that is the combinations of motors, springs andcontrollable dampers used in these embodiments are specifically adaptedto perform the functions needed, a variety of techniques may be employedto automatically control the motor and controllable dampers at the timesneeded to perform the functions illustrated, and any suitable controlmechanism may be employed. FIG. 29 depicts the general form of a typicalcontrol mechanism in which a multiple sensors are employed to determinethe dynamic status of the skeletal structure and the components of thehybrid actuator and deliver data indicative of that status to aprocessor seen at 2900 which produces control outputs to operate themotor actuator and to control the variable dampers.

The sensors used to enable general actuator operation and control caninclude:

-   -   (1) Position sensors seen at 2902 in FIG. 29 located at the        ankle joint axis to measure joint angle (a rotary        potentiometer), and at the motor rotor to measure total        displacement of the motor's drive shaft (as indicated at 2904)        and additionally the motor's velocity (as indicated at 2906). A        single shaft encoder may be employed to sense instantaneous        position, from which motor displacement and velocity may be        calculated by the processor 2900.    -   (2) A force sensor (strain gauges) to measure the actual torque        borne by the joint as indicated at 2908.    -   (3) Velocity sensors on each of the dampers (rotary encoders) as        indicated at 2910 in order to get a true reading of damper        velocity.    -   (4) A displacement sensor on each spring (motor series spring        and global damper spring) as indicated at 2912 in order to        measure the amount of energy stored.    -   (5) One or more Inertial Measurement Units (IMUs) seen at 2914        which can take the form of accelerometers positioned on skeletal        members from which the processor 2900 can compute absolute        orientations and displacements of the artificial joint system.        For example, the IMU may sense the relative vertical movement of        the foot member relative to its foot flat position during the        walking cycle to control foot orientation as discussed above.    -   (6) One or more control inputs manipulatable by a person, such a        wearer of a prosthetic joint or the operator of a robotic        system, to control such things as walking speed, terrain        changes, etc.

The processor 2900 preferably comprises a microprocessor which iscarried on the ankle-foot system and typically operated from the samebattery power source 2920 used to power the motor 2930 and thecontrollable dampers 2932 and 2934. A non-volatile program memory 2941stores the executable programs that control the processing of the datafrom the sensors and input controls to produce the timed control signalswhich govern the operation of the actuator motor and the dampers. Anadditional data memory seen at 2942 may be used to supplement theavailable random access memory in the microprocessor 2900.

Instead of directly measuring the deflection of the motor series springsas noted at (4) above, sensory information from the position sensors (1)can be employed. By subtracting the ankle joint angle from the motoroutput shaft angle, it is possible to calculate the amount of energystored in the motor series spring. Also, the motor series springdisplacement sensor can be used to measure the torque borne by the jointbecause joint torque can be calculated from the motor series outputforce.

Many variations exist in the particular sensing methodologies employedin the measurement of the listed parameters. Although this specificationdescribes preferred sensing methods, each has the goal of determiningthe energy state of the spring elements, the velocities of interiorpoints, and the absolute movement pattern of the ankle joint itself

REFERENCES

The following published materials provide background informationrelating to the invention. Individual items are cited above by using thereference numerals which appear below and in the citations in curleybrackets.

-   -   {1} Palmer, Michael. Sagittal Plane Characterization of Normal        Human Ankle Function across a Range of Walking Gait Speeds.        Massachusetts Institute of Technology Master's Thesis, 2002.    -   {2} Gates Deanna H., Characterizing ankle function during stair        ascent, descent, and level walking for ankle prosthesis and        orthosis design. Master's thesis, Boston University, 2004.    -   {3} Hansen, A., Childress, D. Miff, S. Gard, S. and Mesplay, K.,        The human ankle during walking implication for the design of        biomimetric ankle prosthesis, Journal of Biomechanics (In        Press).    -   {4} Koganezawa, K. and Kato, I., Control aspects of artifical        leg, IFAC Control Aspects of Biomedical Engineering, 1987,        pp.71-85.    -   {5} Herr H, Wilkenfeld A. User-Adaptive Control of a        Magnetorheological Prosthetic Knee. Industrial Robot: An        International Journal 2003; 30: 42-55.    -   {6} Seymour Ron, Prosthetics and Orthotics: Lower limb and        Spinal, Lippincott Williams & Wilkins, 2002.    -   {7} G. A. Pratt and M. M. Williamson, “Series Elastic        Actuators,” presented at 1995 IEEE/RSJ International Conference        on Intelligent Robots and Systems, Pittsburgh, Pa.,    -   {8} Inman VT, Ralston HJ, Todd F. Human walking Baltimore:        Williams and Wilkins; 1981.    -   {9} Hof. A. L. Geelen B. A., and Berg, Jw. Van den, “Calf muscle        moment, work and efficiency in level walking; role of series        elasticity,” Journal of Biomechanics, Vol 16, No. 7, pp.        523-537, 1983.    -   {10} Gregoire, L., and et al, Role of mono- and bi-articular        muscles in explosive movements, International Journal of Sports        Medicine 5, 614-630.    -   {11} Koganezawa, K. and Kato, I., Control aspects of artifical        leg, IFAC Control Aspects of Biomedical Engineering, 1987,        pp.71-85.    -   {12} U.S. Pat. No. 6,517,503 issued Feb. 11, 203.

CONCLUSION

It is to be understood that the methods and apparatus which have beendescribed above are merely illustrative applications of the principlesof the invention. Numerous modifications may be made by those skilled inthe art without departing from the true spirit and scope of theinvention.

What is claimed is:
 1. A device comprising: a mechanical leg joint; acontrollable actuator to exert a torque about the joint, the actuatorcomprising: a motor; and a spring operatively coupled with the motor; anartificial sensory system comprising at least one sensor selected fromthe group consisting of a position sensor, a velocity sensor, agyroscope, an accelerometer, and a force sensor; and an operativeconnection to the motor and the at least one sensor, wherein the deviceis adapted to store elastic energy during one phase of a gait cycle andto release elastic energy during a subsequent phase of the gait cycle toaugment power output of the joint.
 2. The device of claim 1, wherein themechanical joint comprises an ankle joint.
 3. The device of claim 2,wherein the motor is adapted to deflect the spring during controlleddorsiflexion for later release during powered plantar flexion.
 4. Thedevice of claim 3, wherein the motor is controlled to store springenergy over a longer period of time than the amount of time to releasethe spring energy.
 5. The device of claim 1, further comprising a stopadapted to prevent rotation of the joint beyond a predetermined limit.6. A device comprising: a mechanical ankle joint; a motor adapted toexert a torque about the ankle joint; an artificial sensory systemcomprising at least one sensor selected from the group consisting of aposition sensor, a velocity sensor, a gyroscope, an accelerometer, and aforce sensor; and an operative connection to the motor and the at leastone sensor, wherein the operative connection is adapted to output acontrol sequence for stair or slope descent in which during a swingphase an angle of the ankle joint is reoriented for toe-first contactand in which during a stance phase a damping response is subsequentlyapplied during controlled dorsiflexion.
 7. A device comprising: amechanical ankle joint; an artificial elastic element adapted to storeelastic energy; a motor adapted to exert a torque about the ankle joint;a stop for limiting rotation of the ankle joint; at least one sensor;and an operative connection to the motor and the at least one sensor,wherein the operative connection is adapted to control at least one ofjoint stiffness, equilibrium position, damping, and torque.
 8. Thedevice of claim 7, wherein the elastic element is in series with themotor such that, when the joint is locked, the elastic element can becompressed or stretched by the motor in series to the elastic elementwithout the joint rotating.
 9. The device of claim 7, wherein theelastic element amplifies joint power output during a stance phase. 10.The device of claim 7, wherein the elastic element is adapted to storeenergy over a longer period of time than that during which the energy isreleased.
 11. The device of claim 7, wherein the elastic element isadapted to store energy when the stop is engaged.
 12. The device ofclaim 7, wherein the sensor comprises an inertial measurement unit. 13.The device of claim 7, wherein the operative connection is adapted todetermine elevation and is further adapted to reset determined elevationto zero when non-gravitational acceleration is zero.
 14. The device ofclaim 7, wherein the operative connection is adapted to determineelevation after toe-off
 15. The device of claim 7, wherein the operativeconnection is adapted to control an angle of the ankle joint during aswing phase in response to terrain.
 16. The device of claim 15, whereinthe operative connection controls the ankle angle to provide toe strikewhen the elevation change is negative relative to the elevation when thefoot last left the ground.
 17. The device of claim 7, wherein theoperative connection is adapted to output control signals to adjust theankle joint angle during a swing phase of stair descent.
 18. The deviceof claim 7, further comprising a variable damper operatively connectedto the motor.
 19. The device of claim 18, wherein the elastic element isadapted to store energy when the variable damper is set at asufficiently high damping level.
 20. The device of claim 18, wherein atleast one of the motor and the variable damper are programmable.
 21. Thedevice of claim 7, wherein the stop is unidirectional such thatengagement occurs at a predefined ankle joint position to resistdorsiflexion movement.
 22. The device of claim 7, wherein the stop ispositioned in parallel with the motor.
 23. The device of claim 7,wherein the operative connection is adapted to compute torque based on adisplacement of the elastic element.
 24. The device of claim 7, whereinthe operative connection is adapted to compute motor velocity.
 25. Thedevice of claim 7, wherein the operative connection is adapted tocontrol at least one of joint stiffness, equilibrium position, damping,and torque, which at least one parameter comprises at least one of gaitcycle phase, walking speed, elevation and terrain.
 26. An artificialjoint system comprising: an artificial joint; an artificial actuator foractuating the joint; an artificial sensory system comprising at leastone inertial measurement unit adapted to be positioned on a component ofthe system; and an operative connection adapted to compute absoluteorientation and displacement of the joint system based on informationfrom the sensory system, and to reset operative connection computationsupon the detection of an absence of non-centrifugal, non-gravitationallinear acceleration.
 27. The system of claim 26, wherein the operativeconnection is adapted to determine at least one of velocities ofinterior points of the system and an absolute movement pattern of thejoint.
 28. The system of claim 26, wherein the sensory system furthercomprises at least one position sensor adapted to measure joint angle.29. The system of claim 26, wherein the operative connection is adaptedto distinguish between level ground walking and descending or ascendingone of stairs and a steep incline.
 30. The system of claim 26, whereinthe operative connection is adapted to control at least one of jointstiffness, equilibrium position, damping, and torque, in accordance withat least one parameter determined by the controller, which at least oneparameter comprises at least one of gait cycle phase, walking speed,elevation and terrain.